The light absorption and light transmission characteristics of the tissues and fluids of the eye have been the subjects of study for a number of years. By means of various spectroscopic measuring devices, the absorption and transmission by the parts of the eye of light of a wide range of wavelengths have been determined with accuracy sufficient to permit reproduction of the results.
In order to produce light of a range of wavelengths, from the ultraviolet range through the infrared range, mercury vapor, xenon arc, halogen and laser light sources, among others, have been employed. These light sources were then utilized to determine the specific wavelength ranges in which the individual eye parts absorbed or transmitted light.
Among the findings of these studies was the finding that the transmission of ultraviolet light by the frontal portions of the eyes decreases with age, while the transmission of infrared light by those same portions of the eyes was essentially independent of age.
The use of laser light in ophthalmic treatments is a relatively recent development growing out of such studies, one which began in about 1965 with the use of the ruby laser. The use of the argon laser in ophthalmology began shortly thereafter, followed by the advent of the neodymium-doped yttrium-aluminum garnet laser (commonly known as the Nd:YAG laser).
Both the argon laser and the Nd:YAG have recently been utilized in numerous ophthalmic surgical procedures, among them the treatment of glaucoma and cataracts. With respect to treatment of the former condition, one procedure has involved directing a short, strong laser pulse through the pupil so as to form a hole between the anterior and posterior chambers, thereby equalizing intraocular pressure. In another procedure, a laser pulse aimed at the trabecular meshwork is utilized to open a blocked duct to the canal of Schlemm. Alternatively, the laser pulse is used to make a small mark in the trabecular meshwork, which, in the process of scar formation, pulls the surrounding tissue toward the mark. This tissue stretching causes the small ducts and canals of that region to open into the canal of Schlemm, thus restoring the circulation of fluid within the eye.
Other surgical procedures employing laser light are of great importance. Among them are the opening of the lens capsule and the breaking of membranes in the vitreous. With respect to the latter, it is well-known that capsular opacification is the major complication which occurs after extracapsular extraction of the lens and insertion of a posterior chamber implant. While meticulously clearing and polishing the posterior capsule reduce capsular opacification to some degree, the condition is never fully eliminated. Previous treatment of this secondary membrane condition has been accomplished by capsulotomy with a cytotome or knife, or, when the membranes are thick, by pars planar membranectomy. Both of these procedures require substantial intraocular manipulation and both therefore create the risk of intraocular infection or retinal detachment. Accordingly, the use of a laser as a non-invasive method of managing such secondary membranes is highly desirable. For the same reasons, it is desirable to disrupt other types of membranes, such as those which form behind other intraocular implants, by means of laser treatment.
A danger present in each of the laser surgical procedures discussed above, is that the laser light will penetrate beyond the tissues desired to be treated and will be transmitted to the retina where undesirable photocoagulation damage will occur. This is particularly true in the case of capsulotomies and in the case of the breaking of occluding membranes, whether pupillary or vitreous, wherein the laser beam is directed along the visual axis. In these cases, any damage to the fundus results in permanent vision impairment. Therefore, essentially no risk is tolerable in such procedures.
Thus, an important limitation is placed on the scope of use of ophthalmic laser treatments i.e., the need to avoid any substantial laserinduced damage to the fundus.